1. Field of the Invention
This invention relates to a view finder for a reflex camera, and more particularly to an improvement in an optical system of a view finder for a reflex type photographic camera wherein the light passing through a taking lens thereof is put into the view finder system for focusing images on a focusing plate in the view finder system.
2. Description of the Prior Art
In the conventional reflex type photographic cameras, a focusing plate is located at a position equivalent to the focal plane of the taking lens thereof, i.e., the film plane, and the image focused on the focusing plate is viewed in enlarged scale with a magnifier. As the focusing plate there is used a diffusion plate. The diffusion plate is required to be capable of visualizing an image with high resolution and to have uniform brightness over the whole area thereof. For instance, a diffusion plate such as a glass plate or a plastic plate one surface of which is grounded with fine grains of sand or roughened by chemical etching process is used as the focusing plate.
The above described focusing plate suffers from the defect that the marginal portion of the image focused thereon is dark while the central part thereof is comparatively bright, since the amount of light diffused thereby is markedly small in the marginal portion thereof.
In order to avoid the above defect, it has been known in the art to provide a convergent lens adjacent to the focusing plate to direct the light in the marginal portion of the focusing plate toward the eyepiece of the view finder system. Further, it has also been known to form a Fresnel lens on one surface of the focusing plate to enhance the brightness of the image focused thereon and viewed through the view finder. By using the focusing plate having the Fresnel lens on one face thereof the power of the convergent lens located adjacent to the focusing plate can be lowered or the convergent lens can be eliminated and accordingly the weight of the view finder can be lowered and the height of the view finder portion of the camera can be reduced.
However, the above described view finder employing a focusing plate having a Fresnel lens on one face thereof is disadvantageous in that the quality of the image viewed through the view finder is degraded by a number of concentric circles of surface irregularity which exist on the Fresnel lens and which are observed on the roughened surface of the focusing plate glittering thereon. Further, in this type of view finder, the quality of the image is degraded by boundary lines of a prism attached to the focusing plate for focus adjustment which are formed on the roughened face of the focusing plate and can be observed by the photographer through the view finder. Owing to such noise appearing on the focusing plate, the focus adjustment operation is markedly disturbed.